Initially, pneumatic servomotors such as disclosed in U.S. Pat. No. 3,106,873 were constructed in a manner such that air from the environment directly entered the rear chamber thereof to create the operational pressure needed to operate the master cylinder during a braking application. However, after a period of time during which the brakes were repeatedly applied, because of the contaminants carried by the air, the control valves became inoperative. Consequently, an in-line filter was placed in the air supply conduit to prevent contaminants from being communicated to the valve means. As a result, valve stability was enhanced and the noise caused by the air entering the vacuum chamber attenuated. Unfortunately, use of such an in-line filter arrangement results in an increase in the time interval required to create the desired maximum pressure differential. However, the time interval for creation of the maximum pressure differential was completely adequate until the introduction of emission control devices in which the available vacuum produced at the intake manifold of most vehicles was reduced thereby correspondingly reducing the maximum potential pressure differential. As a result, during brake standardization tests established by the Department of Transportation some vehicles could not stop within the set braking distances.
Keeping in mind that a fast brake application, wherein maximum pressure build-up is needed, will only occur during a panic stop, a servomotor was disclosed in U.S. Pat. No. 3,897,716 having a flow control system wherein during a brake application, environmental air will proportionally bypass the filtering means in the servomotor as a function of the pressure differential developed across the filtering means in response to an input actuation force.